The present invention relates to a multi-functional sensing or measuring system incorporating a multi-functional sensing element both of which are adapted for use in conjunction with air conditioners, driers, cooking equipment and so on for detection of ambient temperature and humidity; that is, a relative humidity.
In general, in order to detect the amount or quantity of water vapor in the surrounding atmosphere, humidity responsive sensing elements have been used and in order to detect temperatures thermocouples and thermistors have been widely used. The humidity responsive sensing elements change their resistance in response to variations in humidity and have been used as a humidity sensor or as a sensor for a humidity control system. As is well known in the art, the humidity responsive sensing elements have been fabricated with metal-oxides such as Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3 and so on which have a high water absorption degree. Meanwhile, for one device, equipment or system it would not suffice to detect only an ambient temperature or humidity, and in almost all cases it is required to detect both an ambient temperature and a relative humidity. For instance, the air conditioning system must control both the room temperature and humidity for providing, for instance, comfortable living and working conditions. As a result, a conventional air conditioning system uses a temperature sensing element or elements and a humidity sensing element or elements and includes two control systems responsive to the outputs from these temperature and humidity sensing elements for controlling the temperature and humidity, respectively. Thus the arrangement of these control systems becomes very complex and consequently the costs of air conditioning systems become high.
Recently various industries have been increasingly systematized, so that there has been a strong demand for a single sensing element capable of detecting both an ambient temperature and a relative humidity, but so far there has not devised or proposed a multi-functional sensing element as described above which can detect both an ambient temperature and a relative humidity with a satisfactory degree of accuracy. As a result, the industrial systematization has been much delayed.
The reason why the development of such multifunctional sensing elements of the type described above is difficult is as follows. First, the sensing elements are exposed to the air which contains not only the water vapor but also various components. Some of the components in the air will inevitably cause physical and chemical changes of the sensing elements, and others will adhere to the surfaces of the sensing elements, whereby their temperature and humidity detecting capabilities are degraded. Furtheremore, the relative humidity which is dependent upon the temperature of the air varies over a wide range. Almost all materials respond to variations in relative humidity more or less, but it is extremely difficult to provide a humidity sensing element capable of detecting the relative humidity from 0 to 100%. In general, a conventional humidity sensors have been used for detecting the relative humidity of more than 20% at a temperature less than 80.degree. C. In other words, there has not been devised a multi-functional sensing element capable of detecting a relative humidity from 0 to 100% over a wide temperature variation. Thus without the development of materials which are highly responsive to both temperature and humidity variations, multi-functional sensing elements which are highly reliable, dependable and simple in operation cannot be provided.
Much effort has been made in order to develop sensing elements which are highly reliable in operation and which can vary their electrical characteristics with a high degree of accuracy in response to their absorption water vapor. The recent trend toward such sensing elements is the development of metal-oxide ceramic humidity sensing elements which are thermally stable. However, the degradation of metal oxides due to water vapor absorption is inevitable. Nevertheless, since they are thermally stable, they can be easily regenerated or decontaminated by attaching a heating element to the sensing element or by providing a heating element adjacent to the sensing element so that the latter can be selectively subjected to a heat-treatment.
In general, metal-oxides have a low water absorption capacity; that is, an energy capable of absorbing a small amount of water molecules. As a result, the absorbed water molecules are readily freed from the metal-oxides. This property has been used in humidity sensing elements. More specifically, a humidity sensing element made of metal-oxides exhibits a wide range of resistance change at high humidity due to the absorpotion and desorption of water molecules. Thus the relative humidity can be detected in terms of electrical resistance. However, such humidity sensing elements of the type described have been limited in use to the measurements of dew points and the relative humidity higher than 30%.
Metal-oxide humidity sensing elements have been widely used in the form of an aluminum oxide thin film formed by oxidizing the surfaces of an aluminum thin film, in the form of colloidal particles, in the form of a glaze film consisting of metal oxides and glass or in the form of a ceramic. However, the problem of degradation of their sensitivity due to surface contamination has not yet been solved.